I hurt my ankle a month ago-why won't the pain won't go away?

I landed on my ankle wrong during a kickboxing cl about a month ago, but it never looked bruised or swollen. It still hurts off and on depending on how active I am during the day. I have not been back to the gym since and only walk to work 30 mins a day. Should it be taking this long to heal? I really didn't think I hurt it that badly and never saw a doctor since there were no visible signs of damage. What should I do?